AQA GCSE Physics Physics Paper 1 (Higher), June 2024: Question 5

13 marks · Standard Demand difficulty · Short Answer

Calculate the resistance of a coin, explain why dirty coins are not recognized, draw a thermistor circuit symbol, and determine the potential difference across a thermistor in a potential divider circuit.

Practise this question

Question

A multi-part physics question about electrical circuits, domestic electricity, and a thermistor. Figure 7 shows a hand putting a coin into a vending machine. Questions include stating UK mains frequency and potential difference, calculating resistance from power and current, suggesting why dirty coins fail, drawing a thermistor symbol, reading thermistor resistance from a graph (Figure 9), and calculating potential difference in a series circuit containing a 12V supply, a 400 ohm resistor, and a thermistor.
Question text

05 Figure 7 shows a student putting a coin into a vending machine that sells food.

Figure 7

05.1 The vending machine is connected to the mains electricity supply.

What is the frequency and the potential difference of the mains electricity supply in

the UK?

[2 marks]

Frequency = Hz

Potential difference = V

The vending machine identifies the value of the coin by measuring the resistance of

the coin.

05.2 The power dissipated by the coin is 340 mW when the current in the coin is 0.75 A.

Calculate the resistance of the coin.

Use the Physics Equations Sheet.

[4 marks]

Resistance = Ω

05.3 Coins that are dirty are not recognised by the vending machine.

Suggest one reason why.

[1 mark]

Figure 8 shows part of a different circuit that is used to monitor the temperature inside

the vending machine.

Figure 8

05.4 The circuit symbol for a thermistor has not been included.

Draw the circuit symbol for a thermistor in the box below.

[1 mark]

Figure 9 shows how the resistance of the thermistor varies with temperature.

Figure 9

05.5 The cooling system inside the vending machine turns on when the temperature of the

thermistor is above 20 °C.

Determine the potential difference across the thermistor when the temperature

is 20 °C.

Use the Physics Equations Sheet.

[5 marks]

Potential difference = V

Mark scheme

Show the mark scheme The mark scheme provides answers for each part of question 5. Question 05.1 gives 50 Hz and 230 V. Question 05.2 shows the calculation for resistance resulting in 0.60 ohms. Question 05.3 accepts that dirt changes the measured resistance. Question 05.4 shows the resistor symbol with a diagonal line through it. Question 05.5 details steps to find total resistance, current, and potential difference across the thermistor, resulting in 2.0 V.

Question 5

AO /

Question Answers Extra information Mark

Spec. Ref.

05.1 50 (Hz) this order only 1 AO1

4.2.3.1

230 (V) 1

AO /

Spec. Ref.

05.2 340 mW = 0.34 W 1 AO2

4.2.4.1

0.34 = 0.752 × R allow a correct substitution of an 1

incorrectly / not converted value

of P

0.34 allow a correct rearrangement of 1

R = 2 an incorrectly / not converted

0.75

value of P

R = 0.60 (Ω) allow an answer consistent with 1

an incorrectly / not converted

value of P

allow a correct answer given to

more than 2 sf

AO /

Spec. Ref.

05.3 the dirt changes the (measured) 1 AO3

resistance of the coin 4.2.1.3

or

the (measured) resistance is allow the measured resistance

different from the expected does not match the resistance of

resistance (of the coin) a known coin

allow dirt stops charge flow

(through the coin)

allow dirt stops the– current (in – –

the coin)

AO /

Spec. Ref.

05.4 1 AO1

4.2.1.1

– – –

AO /

Spec. Ref.

05.5 RTotal = 400 + 80 (= 480 Ω) 1 AO2

4.2.1.3

4.2.2

12 = I × 480 allow a correct substitution / 1

or rearrangement with RTotal in

12 range 470 - 490 Ω

I =

I = 0.025 (A) allow a correct calculation using 1

RTotal in range 470 - 490 Ω

V = 0.025 × 80 allow a correct substitution using 1

their calculated value of I (using

V = IR) and RTh in range 70 - 90

Ω

V = 2.0 (V) allow a correct calculation using 1

their calculated value of I (using

V = IR) and RTh in range 70 - 90

Ω

OR

total R = 400 + 80 (= 480) (1)

ratio (Th:R) = 80:480 (1) allow a range of RTh between 70

and 90 Ω

ratio = 1:6 (1) allow a correct ratio using a

value of RTh between 70 and 90

Ω

1 allow a correct substitution using

V = ×12 (1)

6 a value of RTh between 70 and

90 Ω

V = 2.0 (V) (1) allow an answer in the range 1.8

16 (V) to 2.2 (V)

Total Question 5 13

How to answer it

Vending Machine Circuits Study Guide

What this question tests

This question assesses your knowledge of mains electricity values, power calculations involving current and resistance, component identification, and analyzing potential divider circuits containing thermistors using graphical data.

Part 05.1

Mains Electricity Supply

✅ Correct Answers

  • Frequency = 50 Hz
  • Potential difference = 230 V

💡 Key Knowledge

Recall core facts about the UK domestic mains electricity supply. Do not confuse this with US values (110 V, 60 Hz).

Allocated marks: 2 marks (1 mark per correct value)
Part 05.2

Calculating Resistance from Power and Current

📐 Step-by-Step Calculation

  1. Convert power to standard units: 340 mW = 340 ÷ 1000 = 0.34 W
  2. Select the correct equation: P = I² × R (from your Physics Equations Sheet)
  3. Substitute values: 0.34 = 0.75² × R
  4. Rearrange the equation: R = 0.34 / (0.75²)
  5. Calculate final answer: R = 0.6044... = 0.60 Ω (to 2 sf)

❌ Common Errors

  • Unit mismatch: Forgetting to convert milliwatts (mW) into watts (W) before starting the calculation.
  • Squaring errors: Forgetting to square the current ( 0.75² ) in the power formula.
Allocated marks: 4 marks
Part 05.3

Effect of Dirt on Resistance

✅ Acceptable Answers

  • "The dirt changes the measured resistance of the coin."
  • "The measured resistance is different from the expected resistance of the coin."
  • "Dirt stops charge flow / current through the coin."

🧠 Exam Technique

This is an application question (AO3). Relate the physical contamination (dirt) back to electrical properties (resistance or current flow) rather than vague statements like "it looks dirty to the machine".

Allocated mark: 1 mark
Part 05.4

Thermistor Circuit Symbol

✅ Correct Drawing

A standard rectangular resistor symbol with a diagonal line passing through it from bottom-left to top-right (or top-left to bottom-right with a flat base extension).

💡 Key Knowledge

Make sure you know standard circuit symbols: a thermistor is a resistor with a temperature-dependent line across it, whereas a LDR is a resistor inside a circle with arrows pointing inwards.

Allocated mark: 1 mark
Part 05.5

Analyzing a Potential Divider Circuit

📐 Step-by-Step Calculation (Method 1: Current approach)

  1. Find thermistor resistance from graph: At 20 °C, the thermistor resistance is 80 Ω (allow 70–90 Ω).
  2. Calculate total resistance: Rtotal = 400 + 80 = 480 Ω
  3. Calculate circuit current: I = V / Rtotal = 12 / 480 = 0.025 A
  4. Calculate potential difference across thermistor: V = I × Rth = 0.025 × 80 = 2.0 V (allow 1.8 V to 2.2 V)

🧠 Exam Technique

Always check graphs carefully using a ruler. For 20 °C, track up to the curve and across to the resistance axis to read 80 Ω accurately. Showing clear working ensures error carried forward (ecf) marks if your initial reading varies slightly.

Allocated marks: 5 marks

Topics

Physics · P2: Electricity

Question and mark scheme from the AQA GCSE Physics examination, Physics Paper 1 (Higher), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.